Thermoelectric materials are the building blocks of two types of devices (G. S. Nolas, J. Sharp, and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments, Springer (2006)):
1-thermoelectric power generators, capable of converting a heat current into usable electric power, and
2-thermoelectric coolers, capable of refrigerating an object by the only use of electric currents.
These devices are technologically very interesting because they are purely solid state based, which means they do not have moving parts, they are free of vibrations, reliable, compact, and light weight. This makes them ideal for aerospace or micro-electronics applications, for example. However, their efficiency is much lower than that of mechanical energy conversion devices. This limits their applicability in more conventional fields, like usual kitchen refrigerators for example.
A direct measure of a material's ability to be part of a thermoelectric device is given by its dimensionless thermoelectric figure of merit, ZT, defined as TσS2/κ, where T is the temperature, σ is the electrical conductivity, S is the Seebeck coefficient, and κ the thermal conductivity of the material, respectively (G. S. Nolas, J. Sharp, and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments, Springer (2006)). The performance of thermoelectric refrigeration and power generation is hindered by the low ZT of currently known materials. Currently, the best bulk thermoelectric materials have ZT values around 1 at room temperature. Nanostructured materials have shown hints of higher room temperature ZT˜2 in two publications (Venkatasubramanian, R. et al., “Thin-film Thermoelectric. Devices with High Room-temperature Figures of Merit”, Nature, Vol 413, 11 Oct. 2001 pp. 597-602; T. C. Harman, P. J. Taylor, M. P. Walsh, B. E. LaForge, Quantum Dot Superlattice Thermoelectric Materials and Devices, Science 297, pp. 2229 (2002)). A general and ambitious goal of thermoelectrics research is to produce a material with ZT>3 at room temperature. Such material would enable thermoelectric refrigeration with efficiencies comparable to those of pressure based refrigeration, thus opening the market for industrial thermoelectric refrigeration.
A smaller but very important application of thermoelectric refrigeration is in cooling hotspots in microelectronics. A hotspot is an area of the device that does not dissipate heat fast enough, so its temperature becomes too high, affecting performance. A solution to this problem is to actively pump the heat out of the area, by means of an integrated thermoelectric microcooler (A. Shakouri, Nanoscale Thermal Transport and Microrefrigerators on a Chip, Proceedings of IEEE, 94, 1613 (2006)). Shakouri A. has investigated this issue experimentally, using SiGe alloy based microrefrigerators on a chip (Proceeding of the IEEE, vol. 94, no 8, 2006, 1613-1638).
There are several problems when trying to make integrated microrefrigerators suitable for electronic hotspot cooling and related applications. First of all, the refrigerator should be monolithically integrated in the system, so that heat can flow well from the hotspot to the refrigerator, and parasitic interface thermal resistances between them are minimized. In the current silicon based industry, this restricts the available materials to just those compatible with Si technology.
Known thermoelectric materials such as Bi2Te3, or III-V semiconductors, are not suitable to be integrated. An obvious material choice for the refrigerator is SiGe alloys. However, it has been shown by Shakouri's group (A. Shakouri, Nanoscale Thermal Transport and Microrefrigerators on a Chip, Proceedings of IEEE, 94, 1613 (2006)) that SiGe is not efficient enough for most hotspot cooling applications, because its room temperature ZT is only around 0.1. It was further shown by modeling that if one could increase the ZT of SiGe alloys to 0.5 at room temperature, the maximum possible cooling in the microrefrigerator would increase from 4 K to at least 15 K, and even up to 25 K, if ZT increase is due to power factor enhancement. According to Shakouri, a 15 degree cooling renders the microcooler much more useful for hotspots (4 is still insufficient), and developing a SiGe based material with ZT˜0.5 at room temperature would represent a revolution in the area of microelectronics.
Thus the problem that the invention has intended to solve has been to find a SiGe based material, monolithically integrable into Si based technology, with ZT˜0.5 at room temperature, suitable for use in microrefrigerators on a chip.
One strategy to enhance materials' ZT is the inclusion of nanoparticles (particles with sizes ranging from 1 to a few tens of nm) into the material, to form a “nanocomposite.” The embedding material is known as the “matrix”, and the nanoparticles are termed the “filler.” This approach was successfully used by Shakouri and collaborators to enhance the ZT of InGaAs alloys, using ErAs nanoparticles as a filler (Kim, W., Zide, J., Gossard, A., Klenov, D., Stemmer, S., Shakouri, A., and Majumdar, A., “Thermal conductivity reduction and thermoelectric figure of merit increase by embedding nanoparticles in crystalline semiconductors,” Phys. Rev. Lett. 96, 045901 (2006)). For the approach to result in enhanced ZT, it appears to be crucial that the nanoparticles blend well into the matrix, without creating defects or dislocations which would negatively affect the electrical conductivity and lower the ZT (A. Shakouri, Nanoscale Thermal Transport and Microrefrigerators on a Chip, Proceedings of IEEE, 94, 1613 (2006)). But even if this is fulfilled, there is no guarantee that the ZT of the nanocomposite will be higher than that of the original matrix. It is not enough to just embed nanoparticles: their size, composition and concentration must be such that their effect on thermal conductivity, electrical conductivity, and Seebeck coefficient does effectively enhance ZT.
US-2006/0102224 has disclosed nanostructured materials made of a matrix wherein inclusions of nanoparticles are dispersed, wherein the matrix and the nanoparticles are of different composition and both are based on Ge and/or Si.
However experimental results have not shown any ZT improvement in SiGe nanostructured materials (A. Shakouri, Proceedings of the IEEE, 94, 1613, 2006).
The invention has solved the problems of the prior art by providing new materials comprising a SiGe matrix with silicide nanoparticles and/or germanide nanoinclusions, like for example nanoparticles dispersed therein.
Chen H. C. et al., Thin Solid Films, 461, 2004, 44-47, discloses a material made of Si/Si0.8Ge0.2 (001) substrate with epitaxial β-FeSi2 nanodots grown on its surface.
Wu W. W. et al., Applied Physics Letters, 83(9), 2003, 1836-1838, discloses a material made of a Si0.7Ge0.3 on (001) Si matrix with NiSi quantum-dot arrays grown thereon.
However, these materials only comprise a surface layer of silicide nanoparticles.